CN114813332B - Pipeline steel pipe ring-welded joint softening fit usability evaluation method based on bending strain - Google Patents
Pipeline steel pipe ring-welded joint softening fit usability evaluation method based on bending strain Download PDFInfo
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Abstract
The invention provides a method for evaluating the soft fit usability of a pipeline steel pipe ring-welded joint based on bending strain, which comprises the steps of detecting the dent degree and the side expansion rate of a tensile surface of a HAZ of a bending sample and a qualified bending sample of a bending strain device and the bending sample of the pipeline steel pipe ring-welded joint, correcting the thickness of the qualified bending sample by the dent degree and the side expansion rate, and correcting the dent degree S 'based on a thickness factor' R And side expansion E' R The bending strain of (a) is related to the softening dependence of the ring welded joint. The invention breaks through the high-cost technical routes of controlling joint softening such as base metal chemical component adjustment, welding process optimization, post-welding heat treatment and the like, accords with the girth welding joint meeting the usability requirement, has no adverse effect on the overall safety of the structure even if the local softening exists to a certain extent, and represents remarkable progress.
Description
Technical Field
The invention relates to the technical field of welding use evaluation methods, in particular to a soft fit usability evaluation method for a pipeline steel pipe ring welded joint based on bending strain.
Background
Welding is a typical unbalanced heating and non-uniform cooling process, where the weld metal solidifies from a liquid state and undergoes an overall solid phase transformation, resulting in a relatively uniform microstructure. However, the welding HAZ region is very complex due to the difference in distance from the weld puddle, resulting in temperature variations during cooling and formation of a non-uniform tissue region with a gradient. For example: the HAZ region is a fusion region (solid-liquid two-phase region), a super-heated region (coarse-grain region), a fine-grain region (normalizing region or fully recrystallized region), a critical region (incomplete recrystallized region) and a subcritical region (recrystallized region) in this order from the welding fusion line position to the outside of the general low-alloy high-strength steel in a rolled state. Each sub-region of the HAZ may be narrow, but may be of disparate organization type and performance and may be the source of preferential failure in a variety of complex service environments. At a temperature of phase transition A c1 In the subcritical region at or above the finish rolling temperature, the base metal in the rolled state is recovered and recrystallized. Although the structure type is not changed, the change of the crystal inner substructure and dislocation morphology can lead to the weakening or disappearance of the rolling deformation strengthening effect in the original state of the steel, and the macroscopic appearance is a certain degree of softening. For high emphasisIn the HAZ region where the peak temperature of the welding heat cycle is higher than the original tempering temperature, the steel also experiences joint softening. Other, such as deformation-strengthened aluminum alloys, have more severe post-weld HAZ softening and are difficult to meet with joint strength without post-weld heat treatment.
In recent years, various industrial fields have had failure cases of in-service structures due to softening of welded joints. For example: in the field of oil and gas transfer lines, breaks in unpredictable longitudinal strain service environments due to joint strength mismatch and HAZ softening occur sequentially in north america and australia. Cracks generally originate in the weld toe region and propagate toward the weld metal or HAZ softened region, and, from the fracture characteristics, are a distinct ductile fracture. While not causing catastrophic accidents, economic losses and social negative effects are not negligible. Post-weld joint softening is an unavoidable inherent feature of structural steel materials, but not all softening can cause fatal damage, if the degree of softening is not severe, together with deformation strengthening (hardening) under tensile strain during service, post-weld joint softening may not adversely affect service of the overall welded structure. Therefore, it is necessary to evaluate the workability of joint softening of important structures, particularly welded structures subjected to tensile strain.
Petroleum and natural gas transfer lines are typically located in the field away from urban and populated areas. The topography and climate environment are complex and harsh, and particularly in the areas with frequent crust fluctuation, frequent geological disasters and large altitude change, the pipeline girth welding joint faces unpredictable longitudinal tensile load, and the probability of potential failure caused by joint softening is increased. In recent years, the design and manufacture of chemical components of pipeline steel pipes are also changed, and with the improvement of production line rolling and on-line cooling equipment and process capability, the trend of ensuring pipeline steel performance by reducing alloy elements and increasing rolling compression ratio is mainstream. Although the microstructure of the pipeline steel base metal is fully refined under the mature and stable process control condition, the overall performance is uniform and stable, and the tendency of softening the subcritical heat affected zone after the welding heat cycle is increased. How to evaluate the weld-after-weld softening of the pipeline pipe and the compliance under the potential tensile strain condition is a current problem to be solved urgently.
Currently, there are some patent documents related to post-weld joint softening of steel and other metallic materials. Such as: application No. 201280061292.9 discloses a steel for large heat input welding, application No. 200610160565.1 discloses a thick steel plate excellent in toughness and small in softening in a weld heat affected zone, application No. 200380107477.X discloses a high-strength steel plate excellent in softening resistance in a weld heat affected zone and good in hole-enlarging and edge-playing properties, and a method for manufacturing the same. The technology can realize the purpose of softening resistance of the welded joint through the optimal design of the chemical components of the steel, particularly the inhibition effect of the alloy elements on the softening of the joint after the welding heat cycle, but the application cost of the alloy elements is increased to a certain extent. Meanwhile, no report is made on the fit-to-usability evaluation of the degree of softening of the commonly and objectively existing post-weld joint. Application number 201610082273.4 discloses a high-strength and toughness steel plate for a crude oil storage tank with low welding crack sensitivity coefficient and high-temperature PWHT softening resistance and a manufacturing method thereof, and the steel plate can reduce the secondary heating softening of a joint in the welding occasion of the crude oil storage tank with the post-welding heat treatment requirement by combining a plurality of alloy systems.
Application number 201810257705.X discloses a method for reducing the softening degree of a laser welding joint of manganese steel in a high-strength plastic product, which mainly reduces the softening degree of the joint by means of reasonably setting welding process parameters, increasing forced cooling of a back weld joint, reducing the width of a heat affected zone and the like in the laser welding process, and belongs to the welding process adjustment category. Application number 201810312059.2 discloses a method and a device for resisting softening of an electroslag welding joint, which mainly spray compressed air to a part easy to soften in a heat affected zone in a welding process to perform directional cooling, shorten the high-temperature residence time after welding, and effectively reduce the softening of the joint. However, the technology is limited by tooling conditions, the implementation cost is increased, and the implementation and softening degree evaluation guidance effect of the on-site construction girth welding process of the pipeline steel pipe is not great. Application number 201310238270.1 discloses a method for improving the softening problem of a high-strength aluminum alloy welded joint, which mainly comprises the steps of carrying out solution treatment and artificial aging on the welded high-strength aluminum alloy joint, eliminating the softening problem of the joint and meeting the strength requirement of the joint. Application number 201810235277.0 discloses a method for reducing the softening of a deformation-reinforced aluminum alloy arc welding joint by mainly reducing the adverse effect of high-temperature welding thermal cycle on the softening of the joint by forced cooling of circulating water during the welding process. Application number 201810258602.5 discloses a method for rapidly determining the softening area of an Al-Mg-Si alloy welded joint, which is mainly used for rapidly positioning the softening area of the joint by a special metallographic etching method.
In summary, the current technology for softening a welded joint generally reduces the softening tendency of the joint in the welding process through the design of material components and manufacturing processes, or improves the softening behavior of the joint through the optimization of the welding process or the post-welding heat treatment for the existing materials, and does not relate to the evaluation of the inherent softening behavior of the joint, especially the fit-to-use evaluation in combination with the potential service environment.
Disclosure of Invention
The invention overcomes the defects in the prior art, the prior art for softening the welded joint does not relate to the inherent softening behavior of the joint, particularly the fit-to-use performance evaluation is carried out by combining with a potential service environment, and the fit-to-use performance evaluation method for softening the pipeline steel pipe ring-welded joint based on bending strain is provided.
The aim of the invention is achieved by the following technical scheme.
A method for evaluating the soft fit of a pipeline steel pipe ring-welded joint based on bending strain is carried out according to the following steps:
step 1, a pipeline steel pipe girth welding joint bending strain device and a bending sample:
the bending strain device of the pipeline steel pipe girth welding joint comprises two supporting blocks, a pressure head, a pressing device and a clamping device, wherein a gap is formed between the two supporting blocks, a bending sample is arranged on the upper surface of the supporting blocks, a potential softened HAZ of the bending sample is arranged at the position where tension strain is most severe under the pressure head, the position of the bending sample is ensured not to deviate in the bending process, the pressure head is arranged opposite to the gap, the pressure head is arranged on the pressing device, the bending sample is pressed down by the pressure head, the clamping device is contacted with the outer wall of the supporting blocks, the purpose of clamping the supporting blocks by the clamping device is achieved, and after an experiment is started, the pressing down of the pressure head and the clamping action of the clamping device are synchronously performed until the bending sample is bent into a U shape;
step 2, detecting the sag and the side expansion rate of the HAZ tensile face of the qualified bending test sample:
taking out the qualified bending sample obtained in the step 1, detecting the dent degree and the side expansion rate of the HAZ tensile face of the qualified bending sample, taking at least 8 qualified bending samples for each evaluation, taking an average value, and taking the dent degree S R The definition is as follows:side expansion rate E R The definition is as follows: />
Wherein B, W is the original thickness and width of the qualified bent sample, s is the concave depth of the HAZ tensile face after bending in the local softening zone, E is the side expansion of the sample in the width direction after bending deformation,
during bending strain, if the local softening area is wider, the softening degree is larger, and the dent depth and the side expansion amount are both larger, however, if the strain hardening behavior of the local softening area of the HAZ is weakened or the initial welding softening influence is counteracted, the dent depth and the side expansion amount are both smaller, so that the dent degree S combined with the thickness and the width factors of the qualified bending sample is smaller R And side expansion rate E R The influence of preferential deformation and strain hardening of the HAZ local softening area of the pipeline pipe ring welding joint can be reflected;
step 3, correcting the thickness of the qualified bending test sample by the dishing degree and the side expansion rate:
degree of sag S of HAZ tensile face of qualified bending sample R And side expansion rate E R The bending core radius R of the arc-shaped end of the pressure head and the thickness B of the sample are related parameters, but the bending core radius R of the arc-shaped end of the pressure head and the thickness B of the sample are considered to have definite quantity relation, so that the dishing degree S is only needed based on the qualified bending sample thickness B R And side expansion rate E R In general, as the thickness B of the acceptable bending specimen and the radius R of the curved core at the curved end of the indenter are increased, the degree of actual strain experienced by the tensile face of the HAZ of the acceptable bending specimen is appropriately reduced, or the degree of conservation is reduced, so that the degree of sag S at maximum wall thickness is reduced R And side expansion rate E R All corrected to 1.0-1.3 times of the minimum wall thickness, the dishing degree S R And side expansion rate E R The correction factors for the wall thickness factors are defined as:i.e. corrected dishing S' R ＝B R ×S R Corrected side expansion coefficient E' R ＝B R ×E R Because of the dishing degree S R Is not greatly influenced by the width factor of the qualified bending sample, and the side expansion rate E R The width factor has been considered, so the dishing degree S R And side expansion rate E R The width factor of the qualified bent sample is not required to be corrected;
step 4, the concavity S 'corrected based on the thickness factor' R And side expansion E' R Bending strain and ring-welded joint softening dependence:
taking into account the preferential strain of the HAZ local softening area of the pipeline steel pipe ring welded joint and the local strain hardening behavior caused by the preferential strain, weakening or counteracting the initial welding softening influence, the qualified bending sample thickness B and the bending core radius R of the arc-shaped end of the pressure head to the dishing degree S R And side expansion rate E R Factors such as influences of different strength levels of materials on plastic deformation capacity in bending process, and the like are integrated with analysis based on a large amount of test data, and are established as follows based on thicknessDishing degree S 'after degree factor correction' R And side expansion E' R The bending strain and ring-welded joint softening dependence formula:
wherein I is sof The method is used for evaluating the actual softening degree of the welded joint when the welded joint bears a tensile load, not only considers the influence of the welding process on the softening of the welded joint, but also considers the counteracting effect of the strain hardening of the HAZ position on the initial softening state of the joint in the tensile load bearing process, and is very similar to the service state of the welded joint of the pipeline steel pipe.
In step 1, the bending angle of the U-bend specimen was 180 °.
In step 1, the bending specimen is a rectangular side bending specimen, the thickness B of the bending specimen is actually the width in the welding direction, the width W of the bending specimen is the original wall thickness of the steel pipe, the tensile face of the bending specimen cannot be provided with scratches and grooves parallel to the bending tensile direction, and the edge of the bending specimen should be smoothly transited to avoid sharp edges.
In step 1, the thickness b=8-20 mm of the bent sample, the radius r=36-90 mm of the bent core of the arcuate segment of the indenter, and r=4.5B is ensured.
In step 2, the criterion for the qualified bent sample is: if a crack exceeding 3mm in length is detected in the HAZ tensile face, the failure of the bent sample is judged.
In step 3, the dishing degree S at the maximum wall thickness is set R And side expansion rate E R Are each corrected to 1.2 times the minimum wall thickness.
In step 4, when I sof >1, it is shown that the ring welded joint does not significantly soften actually when I sof <1, it is shown that significant actual softening of the ring welded joint occurs.
I sof And the dishing degree S 'corrected based on the thickness factor' R And side expansion E' R Has a negative correlation, and has stronger correlation,that is, S' R And E' R The larger I sof The smaller the joint, the more severe the actual softening tendency, I sof Yield strength Y with base metal σ Also has a negative correlation, but the correlation is relatively small, that is, Y σ The larger I sof The smaller the joint, the more serious the actual softening tendency, which is consistent with common knowledge, and the application of the actual softening index I of the pipeline steel pipe girth welding joint shown in the formula (1) is verified by multiple tests sof The evaluation is carried out, and the evaluation result can reflect the actual condition of softening failure fracture of the ring-welded joint which is actually in service and bears the tensile load, and has the characteristics suitable for usability.
The beneficial effects of the invention are as follows: aiming at the fact that the HAZ is softened after the field welding of the low-alloy high-strength pipeline steel pipe, the invention utilizes the dent degree and the side expansion rate of the tensile face of the HAZ after guiding bending and the actual softening index I of the ring-welded joint fitted by the same sof Judging the suitability of the joint for use by local softening, fully considering the characteristics of unpredictable tensile strain born in the actual service environment of the steel pipes with different strength levels and different strain strengthening of the local softening part, being very close to the engineering reality, and having direct guiding effect on the safety evaluation of the pipeline girth welded joint; compared with the prior art, the invention breaks through the high-cost technical routes of controlling joint softening such as base metal chemical component adjustment, welding process optimization, post-welding heat treatment and the like, accords with the ring welding joint meeting the usability requirement, has no adverse effect on the overall safety of the structure even if the ring welding joint is locally softened to a certain extent, and represents remarkable progress; the technology of the invention has general applicability and important application value for other industrial fields similar to joint forms and service occasions; the guide bending test and test evaluation process is simple to operate, convenient to implement and low in overall cost.
Drawings
FIG. 1 is a pipeline steel pipe girth weld joint bending strain apparatus and test specimen;
FIG. 2 is a graph showing the sag and side swell ratio of the HAZ tensile face of a pilot side bending pass specimen.
Detailed Description
The technical scheme of the invention is further described by specific examples.
A method for evaluating the soft fit of a pipeline steel pipe ring-welded joint based on bending strain is carried out according to the following steps:
step 1, a pipeline steel pipe girth welding joint bending strain device and a bending sample:
the bending strain device of the pipeline steel pipe girth welding joint comprises two supporting blocks, a pressure head, a pressing device and a clamping device, wherein a gap is formed between the two supporting blocks, a bending sample is arranged on the upper surface of the supporting block, a HAZ with potential softening of the bending sample is arranged at the position with the most severe tensile strain under the pressure head and is arranged at the gap, the bending sample position is ensured not to deviate in the bending process, the pressure head is arranged opposite to the gap, the pressure head is arranged on the pressing device, the bending sample is pressed down by the pressure head, the clamping device is contacted with the outer wall of the supporting block, after the experiment is started, the pressing down of the pressure head and the clamping action of the clamping device are synchronously performed until the bending sample is bent into a U shape, the bending angle of the U-shaped bending sample is 180 degrees, the bending sample is a rectangular side bending sample, the thickness B of the bending sample is actually the width in the welding direction, the width W of the bending sample is the original wall thickness of a steel pipe, the tensile face of the bending sample cannot be provided with scratches and grooves parallel to the bending and stretching direction, the edge of the bending sample is smoothly transited to avoid sharp edges, the thickness B=8-20 mm of the bending sample, the bending core radius R=36-90 mm of the arc section of the pressure head and R=4.5B are ensured;
step 2, detecting the sag and the side expansion rate of the HAZ tensile face of the qualified bending test sample:
taking out the qualified bending sample obtained in the step 1, wherein the judging standard of the qualified bending sample is as follows: if a crack with the length exceeding 3mm is detected on the HAZ tensile face, judging that the bending sample is unqualified, detecting the dent degree and the side expansion rate of the HAZ tensile face aiming at the qualified bending sample, taking at least 8 qualified bending samples for each evaluation, averaging, and taking the dent degree S R Definition of the definitionThe method comprises the following steps:side expansion rate E R The definition is as follows: />
Wherein B, W is the original thickness and width of the qualified bent sample, s is the concave depth of the HAZ tensile face after bending in the local softening zone, E is the side expansion of the sample in the width direction after bending deformation,
during bending strain, if the local softening area is wider, the softening degree is larger, and the dent depth and the side expansion amount are both larger, however, if the strain hardening behavior of the local softening area of the HAZ is weakened or the initial welding softening influence is counteracted, the dent depth and the side expansion amount are both smaller, so that the dent degree S combined with the thickness and the width factors of the qualified bending sample is smaller R And side expansion rate E R The influence of preferential deformation and strain hardening of the HAZ local softening area of the pipeline pipe ring welding joint can be reflected;
step 3, correcting the thickness of the qualified bending test sample by the dishing degree and the side expansion rate:
degree of sag S of HAZ tensile face of qualified bending sample R And side expansion rate E R The bending core radius R of the arc-shaped end of the pressure head and the thickness B of the sample are related parameters, but the bending core radius R of the arc-shaped end of the pressure head and the thickness B of the sample are considered to have definite quantity relation, so that the dishing degree S is only needed based on the qualified bending sample thickness B R And side expansion rate E R In general, as the thickness B of the acceptable bending specimen and the radius R of the curved core at the curved end of the indenter are increased, the degree of actual strain experienced by the tensile face of the HAZ of the acceptable bending specimen is appropriately reduced, or the degree of conservation is reduced, so that the degree of sag S at maximum wall thickness is reduced R And side expansion rate E R All corrected to 1.2 times of the minimum wall thickness, the dishing degree S R And side expansion rate E R The correction factors for the wall thickness factors are defined as:i.e. corrected dishing S' R ＝B R ×S R Corrected side expansion coefficient E' R ＝B R ×E R Because of the dishing degree S R Is not greatly influenced by the width factor of the qualified bending sample, and the side expansion rate E R The width factor has been considered, so the dishing degree S R And side expansion rate E R The width factor of the qualified bent sample is not required to be corrected;
step 4, the concavity S 'corrected based on the thickness factor' R And side expansion E' R Bending strain and ring-welded joint softening dependence:
taking into account the preferential strain of the HAZ local softening area of the pipeline steel pipe ring welded joint and the local strain hardening behavior caused by the preferential strain, weakening or counteracting the initial welding softening influence, the qualified bending sample thickness B and the bending core radius R of the arc-shaped end of the pressure head to the dishing degree S R And side expansion rate E R Factors such as influence of different strength levels of materials on plastic deformation capacity in bending process, and the like, are integrated with analysis based on a large amount of test data, and the following sag S 'corrected based on thickness factors is established' R And side expansion E' R The bending strain and ring-welded joint softening dependence formula:
wherein I is sof The method is used for evaluating the actual softening degree of the welded joint when the welded joint bears a tensile load, not only considers the influence of the welding process on the softening of the welded joint, but also considers the counteracting effect of the strain hardening of the HAZ position on the initial softening state of the joint in the tensile load bearing process, and is very similar to the service state of the welded joint of the pipeline steel pipe.
When I sof >1, it is shown that the ring welded joint does not significantly soften actually when I sof <1, it is shown that significant actual softening of the ring welded joint occurs.
I sof And the dishing degree S 'corrected based on the thickness factor' R And side expansion E' R Has a negative correlation and a strong correlation, that is, S' R And E' R The larger I sof The smaller the joint, the more severe the actual softening tendency, I sof Yield strength Y with base metal σ Also has a negative correlation, but the correlation is relatively small, that is, Y σ The larger I sof The smaller the joint, the more serious the actual softening tendency, which is consistent with common knowledge, and the application of the actual softening index I of the pipeline steel pipe girth welding joint shown in the formula (1) is verified by multiple tests sof The evaluation is carried out, and the evaluation result can reflect the actual condition of softening failure fracture of the ring-welded joint which is actually in service and bears the tensile load, and has the characteristics suitable for usability.
Aiming at the method for evaluating the usability of the pipeline steel pipe girth weld joint soft compound based on bending strain, the joint softening evaluation technology is implemented under different welding heat input conditions based on X70 pipeline steel commonly used for petroleum and natural gas long-distance pipelines. Table 1 shows the actual yield strength Y of the parent metal of the pipeline steel pipe in the examples of the evaluation of the flexibility of the girth weld joint based on bending strain for 5 different welding heat inputs X70 (25.4 mm) pipeline steel pipes σ =535 MPa; here, the acceptable bent sample thickness b=10 mm, r=45 mm, and the sample width W (steel pipe wall thickness) =25.4 mm; thus, the concavity SR and the side expansion ER are both corrected by a factor br=1.033 with respect to the wall thickness factor; using the actual softening index I of the girth welded joint sof Indirectly evaluating the actual softening degree of the girth welded joint of different embodiments after the longitudinal tensile strain of the steel pipe; it can be seen that the softening of the joints of examples 1-3 does not adversely affect the overall structural service of the pipeline conduit, or, in other words, the joints do not exhibit substantial softening through simulating the tensile strain of the pipeline conduit girth weld joint; example 4 shows a slight softening, the actual softening index I of the girth welded joint sof Under the condition of being very close to a critical value, in practical application, the local softening is considered to have little influence on the overall service safety of the structure; example 5 the ring welded joint exhibited significant physical softening, and it was believed that the joint was partially softened and the structure was entirely in serviceThe safety has a certain influence.
Table 1 5 different weld heat input X70 pipe joints soft-compound on bending strain evaluation examples
Remarks: GMAW means consumable electrode gas shielded automatic welding, SMAW means low hydrogen electrode manual welding, and SAW means submerged arc automatic welding.
The foregoing has described exemplary embodiments of the invention, it being understood that any simple variations, modifications, or other equivalent arrangements which would not unduly obscure the invention may be made by those skilled in the art without departing from the spirit of the invention.
Claims (6)
1. A method for evaluating the softening and usability of a pipeline steel pipe ring-welded joint based on bending strain is characterized by comprising the following steps: the method comprises the following steps of:
step 1, a pipeline steel pipe girth welding joint bending strain device and a bending sample:
the bending strain device of the pipeline steel pipe girth welding joint comprises two supporting blocks, a pressure head, a pressing device and a clamping device, wherein a gap is formed between the two supporting blocks, a bending sample is arranged on the upper surface of the supporting blocks, a potential softened HAZ of the bending sample is arranged at the position where tension strain is most severe under the pressure head, the position of the bending sample is ensured not to deviate in the bending process, the pressure head is arranged opposite to the gap, the pressure head is arranged on the pressing device, the bending sample is pressed down by the pressure head, the clamping device is contacted with the outer wall of the supporting blocks, the purpose of clamping the supporting blocks by the clamping device is achieved, and after an experiment is started, the pressing down of the pressure head and the clamping action of the clamping device are synchronously performed until the bending sample is bent into a U shape; the bending sample is a rectangular side bending sample, the thickness B of the bending sample is actually the width in the welding direction, the width W of the bending sample is the original wall thickness of the steel pipe, the tensile face of the bending sample cannot be provided with scratches and grooves parallel to the bending and stretching direction, and the edge of the bending sample should be smoothly transited to avoid sharp edges;
step 2, detecting the sag and the side expansion rate of the HAZ tensile face of the qualified bending test sample:
taking out the qualified bending sample obtained in the step 1, detecting the dent degree and the side expansion rate of the HAZ tensile face of the qualified bending sample, taking at least 8 qualified bending samples for each evaluation, taking an average value, and taking the dent degree S R The definition is as follows:side expansion rate E R The definition is as follows: />
Wherein B, W is the original thickness and width of the qualified bent sample, s is the concave depth of the HAZ tensile face after bending in the local softening zone, E is the side expansion of the sample in the width direction after bending deformation,
the judging standard of the qualified bending sample is as follows: if the HAZ tensile face is detected to have cracks with the length exceeding 3mm, judging that the bending sample is unqualified; during bending strain, if the local softening area is wider, the softening degree is larger, and the dent depth and the side expansion amount are both larger, however, if the strain hardening behavior of the local softening area of the HAZ is weakened or the initial welding softening influence is counteracted, the dent depth and the side expansion amount are both smaller, so that the dent degree S combined with the thickness and the width factors of the qualified bending sample is smaller R And side expansion rate E R The influence of preferential deformation and strain hardening of the HAZ local softening area of the pipeline pipe ring welding joint can be reflected;
step 3, correcting the thickness of the qualified bending test sample by the dishing degree and the side expansion rate:
degree of sag S of HAZ tensile face of qualified bending sample R And side expansion rate E R The method is characterized in that the method belongs to the related parameters of the thickness B of the sample and the radius R of the curved end of the pressure head, but takes the fact that the thickness B of the sample and the radius R of the curved end of the pressure head have definite quantity relationThus, the dishing degree S is only required to be based on the thickness B of the qualified bending sample R And side expansion rate E R In general, as the thickness B of the acceptable bending specimen and the radius R of the curved core at the curved end of the indenter are increased, the degree of actual strain experienced by the tensile face of the HAZ of the acceptable bending specimen is appropriately reduced, or the degree of conservation is reduced, so that the degree of sag S at maximum wall thickness is reduced R And side expansion rate E R All corrected to 1.0-1.3 times of the minimum wall thickness, the dishing degree S R And side expansion rate E R The correction factors for the wall thickness factors are defined as:i.e. corrected dishing S' R ＝B R ×S R Corrected side expansion coefficient E' R ＝B R ×E R Because of the dishing degree S R Is not greatly influenced by the width factor of the qualified bending sample, and the side expansion rate E R The width factor has been considered, so the dishing degree S R And side expansion rate E R The width factor of the qualified bent sample is not required to be corrected;
step 4, the concavity S 'corrected based on the thickness factor' R And side expansion E' R Bending strain and ring-welded joint softening dependence:
taking into account the preferential strain of the HAZ local softening area of the pipeline steel pipe ring welded joint and the local strain hardening behavior caused by the preferential strain, weakening or counteracting the initial welding softening influence, the qualified bending sample thickness B and the bending core radius R of the arc-shaped end of the pressure head to the dishing degree S R And side expansion rate E R Factors such as influence of different strength levels of materials on plastic deformation capacity in bending process, and the like, are integrated with analysis based on a large amount of test data, and the following sag S 'corrected based on thickness factors is established' R And side expansion E' R The bending strain and ring-welded joint softening dependence formula:
wherein I is sof The method is used for evaluating the actual softening degree of the welded joint when the welded joint bears a tensile load, not only considering the influence of the welding process on the softening of the welded joint, but also considering the counteracting effect of the strain hardening of the HAZ position on the initial softening state of the joint in the tensile load bearing process, and is very similar to the service state of the welded joint of the pipeline steel pipe ring; when I sof >1, it is shown that the ring welded joint does not significantly soften actually when I sof <1, it is shown that significant actual softening of the ring welded joint occurs.
2. The method for evaluating the soft fit of the ring welded joint of the pipeline steel pipe based on bending strain according to claim 1, wherein the method comprises the following steps of: in step 1, the bending angle of the U-bend specimen was 180 °.
3. The method for evaluating the soft fit of the ring welded joint of the pipeline steel pipe based on bending strain according to claim 1, wherein the method comprises the following steps of: in step 1, the thickness b=8-20 mm of the bent sample, the radius r=36-90 mm of the bent core of the arcuate segment of the indenter, and r=4.5B is ensured.
4. The method for evaluating the soft fit of the ring welded joint of the pipeline steel pipe based on bending strain according to claim 1, wherein the method comprises the following steps of: in step 3, the dishing degree S at the maximum wall thickness is set R And side expansion rate E R Are each corrected to 1.2 times the minimum wall thickness.
5. The method for evaluating the soft fit of the ring welded joint of the pipeline steel pipe based on bending strain according to claim 1, wherein the method comprises the following steps of: i sof And the dishing degree S 'corrected based on the thickness factor' R And side expansion E' R Has a negative correlation and a strong correlation, that is, S' R And E' R The larger I sof The smaller the joint, the more severe the actual softening tendency, I sof Yield strength Y with base metal σ Also has a negative correlation, but the correlation is relatively small, that is, Y σ The larger I sof The smaller the joint, the more serious the actual softening tendency, which is consistent with common knowledge, and the application of the actual softening index I of the pipeline steel pipe girth welding joint shown in the formula (1) is verified by multiple tests sof The evaluation is carried out, and the evaluation result can reflect the actual condition of softening failure fracture of the ring-welded joint which is actually in service and bears the tensile load, and has the characteristics suitable for usability.
6. Use of a bending strain based pipeline steel pipe girth weld joint softening adapted to usability assessment method as claimed in any one of claims 1 to 5 to reflect the impact of low carbon micro alloyed pipeline steel pipe joint softening on structural integrity under actual service conditions.
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